1. Field of the Invention
The present invention relates to a zoom lens system which is suitable for use in a broadcasting television camera, a video camera, a digital still camera, and a silver-halide camera, and also to an image pickup apparatus including the zoom lens system.
2. Description of the Related Art
Conventionally, as a zoom lens system used for an image pickup apparatus such as a television camera, a silver halide film camera, a digital camera, and a video camera, there is proposed a zoom lens system including, in order from the object side, a first lens unit having positive refractive power and including a focus unit, a second lens unit having negative refractive power for magnification-varying, a third lens unit having positive or negative refractive power for correcting image plane variation due to the magnification-varying, and a fourth lens unit having positive refractive power for image formation.
Japanese Patent Application Laid-Open No. 2004-264458 discloses a zoom lens system in which the third lens unit has a positive refractive power. Japanese Patent Application Laid-Open No. H09-005628 discloses a zoom lens system in which the third lens unit has a negative refractive power.
Japanese Patent Application Laid-Open No. 2004-264458 discloses a technology for appropriately correcting longitudinal chromatic aberration in particular by defining a refractive index, an Abbe constant, a shape, or the like of the first lens.
Japanese Patent Application Laid-Open No. H09-005628 discloses a technology for realizing a zoom lens system with small spherical aberration and chromatic aberration in particular by defining a structure of positive and negative lenses in the first lens unit and an Abbe constant thereof.
However, there is a problem that if it is intended to correct longitudinal chromatic aberration due to an increase in a focal length at a telephoto end because of a higher magnification ratio or to correct further longitudinal chromatic aberration on the telephoto side in the already disclosed zoom lens system, it is difficult to correct the longitudinal chromatic aberration sufficiently by the conventional technology.